Various types of body-implantable medical leads are known and used within the medical field. For example, implantable medical devices (IMDs), such as pacemakers, cardiac defibrillators and cardioverters, are, in operation, connected to implantable medical leads for sensing cardiac function and other diagnostic parameters and delivering stimulation pulses.
Implantable medical leads can broadly be divided into two different groups depending on the fixation and anchoring to tissue in the subject's body. A first group includes implantable medical leads of the so-called passive fixation type. Such an implantable medical lead comprises a physical structure close to the distal end of the implantable medical lead. Following implantation the implantable medical lead and in particular the distal end that is brought into contact with the target tissue in the subject's body will be covered by a fibrin clot and connective tissue. The physical structure is then entangled in the connective tissue and is thereby anchored to the target tissue. Examples of such physical structures employed in the art of implantable medical leads of the passive fixation type include collar, tines and fins.
The second group includes implantable medical leads of the so-called active fixation type. These implantable medical leads comprise a fixation structure that is actively fixed and anchored to the target tissue. The fixation structure is typically in the form of a helix or screw-like element that is actively screwed into the target tissue to thereby anchor the implantable medical lead.
Today implantable medical leads of the active fixation type are most common due to, among others, more reliable tissue fixation. Though generally preferred over the passive fixation type these implantable medical leads may have their shortcomings and disadvantages. FIGS. 10A and 10B are cross-sectional views of an example of the distal portion of a prior art implantable medical lead of the active fixation type. In order to extend the fixation helix 122 into the target tissue a torque is applied to a connector pin at the opposite, proximal portion of the implantable medical lead. The applied torque causes, due to a mechanical connection between the connector pin and a conductor coil 142, rotation of the conductor coil 142 and a shaft 126 mechanically connected to the conductor coil 142. This shaft 126 in turn interconnects the fixation helix 122 to the conductor coil 142 and thereby causes a rotation of the fixation helix 122 when the conductor coil 142 is rotated. Rotation of the fixation helix 122 is translated into a longitudinal movement of the fixation helix 122 out of the distal portion of the implantable medical lead through the action of a post 121 protruding between adjacent turns of the fixation helix 122.
In order to prevent unintentional movement of the fixation helix 122 out of the distal portion, e.g. during implantation of the implantable medical lead through the vascular system of the subject, a contact spring 150 is provided between a shoulder 154 of a fixed coupling 128 and a stop structure 152 attached to the outer or lateral surface of the shaft 126. The contact spring 150 further defines the final helix extension length as illustrated in FIG. 10B, where this final helix extension length is reached when the contact spring 150 is fully compressed between the shoulder 154 and the stop structure 152. A further function of the contact spring 150 is to establish electrical contact between the shaft 126 and the coupling 128 in order to eliminate chatter problems that can occur between the shaft 126 and the coupling 128 due to a free floating potential in the coupling 128 relative the shaft 126, the fixation helix 122 and the conductor coil 142.
However, the contact spring 150 may create an increased friction between the post 121 and the fixation helix 122. This can be experienced as a non-repeatable helix performance since the induced friction can vary from one implantable medical lead to another implantable medical lead. Another issue is the risk of jumpiness during extension and retraction of the fixation helix 122 and the risk of a jammed but not fully extended fixation helix 122. These problems can be caused by the end of the contact spring 150 getting into or at least trying to get into the space between the coupling 128 and the shaft 126. Furthermore, as mentioned above, the final helix extension length is dictated by the compression of the contact spring 150. The compression of the contact spring 150 may, however, vary from contact spring to contact spring due to small variations in wire diameter, number of turns and spring end configurations. Furthermore, in some lead designs, compressed turns of the contact spring 150 could sometimes become stuck on top of each other or become slightly offset relative each other. This can make the final helix extension length unreliable and differ from one implantable medical lead to another implantable medical lead.
There is, thus, a need for improvements to implantable medical leads of the active fixation type.